Patent GB-798629-A (GB)
Title: Electrical high frequency transmission line filter arrangements
Inventor: DUKES JOHN MARCUS CUTHBERT
IPC: H01P1/213
Filed: 1955-09-01
Abstract: 798,629. Filters. STANDARD TELEPHONES & CABLES, Ltd. July 4,1956 [Sept. 1, 1955], No. 25095/55. Class 40 (8). A filter comprises several strip transmission lines connected in a series chain each comprising two transmission line branches of unequal length mutually coupled at both ends. The strip line may be of any of the forms shown in Figs. 1a to 1d. As shown in Fig. 3, three networks 8, 9, 10 are inserted in a main transmission line 11, 11<SP>1</SP> of the type shown in Fig. 1b having a ground plate 14, a thin slab of dielectric material 15, e.g. polytetrafluoroethylene impregnated glass fibre, and the conductors 11, 12, 13, 11<SP>1</SP>. The conductors may be formed by a photo etching printing process. To prevent the effect of energy changes of the fringe fields, the filter may be enclosed in a metal box or surrounded by a wire mesh screen. Alternatively, the impedance level of the system may be reduced so that a larger proportion of the energy is confined to the space between the conductor and the ground plane by (a) reducing the thickness of the dielectric slab, (b) increasing the width of the strip conductors, (c) increasing the dielectric constant of the slab, or (d) where two ground planes are used, joining them together. A construction is shown in Fig. 5 using the strip line shown in Fig. 1d and comprising networks of different lengths 16 to 28. To interchange the stop and pass bands of a filter, a frequency insensitive phase reversal may be effected by forming complementary slots 48 in the conductors 45, Fig. 9a, and providing conducting pins 50, 51. The networks may be mutually coupled in parallel as shown or in series (see Fig. 10, not shown). A series junction network is shown in Fig. 12 where conductors 68, 68a are carried on each side of a sheet 69 of dielectric material which is clamped at the edges between the side walls of two ground plane conductors 70, 70a. Thin copper shins 72 ensure uniform non-resonant connection of the conductors 70, 70a. The network branches being obtained by providing conducting septa 73, 73a extending longitudinally one line comprising the conductors 70 and 70a together with the outer surfaces of the septa while the other branch line comprises the inner surfaces of the septa with the conductors 68, 68a. The electrical length of this latter branch being made greater by filling the space enclosed by the septa with a low-loss material 74. Spurious responses in the filter may be reduced by providing a small amount of inherent dissipation, e.g. by using a slightly lossy dielectric, e.g. glass fibre impregnated with a silicone resin. Alternatively the resistance of the conductors may be increased by plating the surfaces with a layer of metal such as tin or rhodium or the conductors may be made wholly from tin or aluminium which forms a low conductivity skin by oxidation. The filters are shown in a circuit (see Fig. 13, not shown) for separating two frequency bands. This circuit may be used for single frequency band operation as a constant impedance equalizer or filter by making the filters of the same pass band and applying the output of one of the filters to a dummy load, e.g. a tapered card of lossy material. The Specification contains a mathematical discussion and explains the procedure for designing the filter. Specifications 491,359 and 579,414, [both in Group XL], are referred to.